Purification of unsaturated product

ABSTRACT

Methyl Chloride and vinyl chloride can be essentially eliminated from a butadiene stream derived from an oxidative dehydrogenation of n-butenes in the presence of chlorine by first separating the butadiene from the higher boiling C4&#39;&#39;s and concentrating the methyl chloride and vinyl chloride in the butadiene fraction by fractionation then fractionating the butadiene fraction to produce a concentrate of methyl chloride and vinyl chloride as an overhead and producing a butadiene product essentially free of chlorides. The loss in butadiene based on the initial feed to this purification is less than 0.15 percent volume. The use of chlorine in the oxidative dehydrogenation which necessitates the purification produces 5 to 15 mole percent absolute more butadiene than the same process in the absence of chlorine thus justifying the purification.

United States Patent Tschopp 1 March 6, 1973 Primary ExaminerDelbert E.Gantz Assistant ExaminerVeronica OKeefe Attorney-G. Baxter Dunaway [57]ABSTRACT Methyl Chloride and vinyl chloride can be essentiallyeliminated from a butadiene stream derived from an oxidativedehydrogenation of n-butenes in the presence of chlorine by firstseparating the butadiene from the higher boiling C s and concentratingthe methyl chloride and vinyl chloride in the butadiene fraction byfractionation then fractionating the butadiene fraction to produce aconcentrate of methyl chloride and vinyl chloride as an overhead andproducing a butadiene product essentially free of chlorides. The loss inbutadiene based on the initial feed to this purification is less than0.15 percent volume. The use of chlorine in the oxidativedehydrogenation which necessitates the purification produces 5 to 15mole percent absolute more butadiene than the same process in theabsence of chlorine thus justifying the purification.

5 Claims, 1 Drawing Figure [75] Inventor: Lloyd D. Tschopp, l-lumble,Tex.

[73] Assignee: Petro-Tex Chemical Corporation,

Houston, Tex.

[22] Filed: Sept. 30, 1970 [21] Appl. No.: 76,751

[52] US. Cl. ..260/680 D, 260/6815 R, 203/81 [51] Int. Cl ..C07c 11/16,C07c 7/04 [58] Field of Search ..260/68 1 .5 R, 680 D [56] ReferencesCited UNITED STATES PATENTS 3,327,001 6/l967 Tschopp ..260/68l.5 R3,402,215 9/1968 Woerner et al. ..260/680 R 3,4l2,l7l ll/l968 Welch etal. ....260/68l.5 3,474,155 10/1969 Tschopp et al ..260/677 ca/vrz/vr/mr:

a0 TAD/[A f Final/r r ample, U.S. Pats. Nos. 3,211,800; 3,268,611;3,268,612; 3,274,285; 3,277,207; 3,303,234; 3,306,450; 3,308,182;3,308,184; 3,308,197-200;

3,316,320; 3,359,343; 3,420,912; 3,440,298; and 3,442,968 as well asnumerous other U.S. and foreign patents. According to these patents,unsaturated hydrocarbons such as butadiene may be produced by reacting amixture of the compound to be dehydrogenated, e.g., butene-2, oxygen anda source of chlorine at an elevated temperature. The effluent from thedehydrogenation zone or reactor comprises the unsaturated product, someunconverted feed, CO water, possibly inert diluents (particularly if theoxygen source is air), some inorganic chloride and organic chloridessuch as methyl chloride and vinyl chloride. Although these organicchlorides are present in relatively small amounts, they are seriouscontaminates in the product and are extremely difficult to remove. Forexample, methyl chloride in the butadiene significantly affects theutility of the product because of the corrosive nature of organicchlorides and because methyl chloride may affect the polymerization rateof the monomer, as well as have possible adverse effect on thepolymerization catalyst.

The principal use of butadiene is the preparation of synthetic rubbersuch as copolymers of butadiene and styrene and butadiene andacrylonitrile. The synthesis requires an extremely pure butadiene withas little deleterious material as possible remaining therein. Forexample, styrene-butadiene rubber (SBR) is commercially prepared by lessthan a complete conversion of monomers to polymer, with the unreactedbutadiene being recovered an recycled in the polymerization. Recycle ofthe butadiene causes a rapid buildup of organic chloride impurities thusmaking even low levels of contamination impractical.

Furthermore, the organic chloride represents a portion of the catalystand in a commercial operation the loss of even a small percentage of thecatalyst can radically affect the economic aspects of the process. .Inany event the organic chlorides are valuable dehydrogenation catalystand should be recovered from the product for purification and recycle tothe dehydrogenation zone.

The removal of water soluble inorganic halides such as HCl and Nl-Lclfrom the reactor effluent is different from that of the removal oforganic chlorides. One distinction which has been found is that theorganic chlorides behave similarly to the organic dehydrogenationproduct and as a result cannot be washed out with water. When ahydrocarbon effluent contaminated with methyl chloride and vinylchloride is washed with water essentially all of the organic chloridepass through the water along with the washed hydrocarbon.

The problem of organic chloride removal is aggravated further by therelatively small amounts present based on the other possible componentsof the effluent such as unreacted feed, products such as monolefins anddiolefins, steam, nitrogen, oxygen and decomposition products. Theorganic chloride may amount to only a few tenths percent or less of theeffluent.

Several methods for separating organic halogen compounds fromhydrocarbon streams are known, for example, it has been proposed to useselective solvents in extractive distillation or to react thehydrocarbon halide with an excess of ammonia to form a water solublereaction product. lt was known in the prior that it is particularlydifficult to separate hydrocarbon halides, such as methyl chloride ormethyl bromide, from such compounds as n-butane and butene-2 bydistillation.

It has now been found that chloride impurities can be essentiallycompletely removed from a butadiene containing stream by a particulararrangement of distillations. Briefly stated the present invention is aprocess for separating hydrocarbon chlorides from a mixture containingbutadiene and hydrocarbon chlorides comprising passing said mixture to afractionator, withdrawing a bottom fraction essentially free ofhutadiene and hydrocarbon chloride, withdrawing a first overheadcomprising butadiene and hydrocarbon chloride, said first overheadhaving a higher concentration of hydrocarbon chloride than said mixturepassing said first overhead to a second fractionator, withdrawing asecond overhead, said second overhead having a substantially higherconcentration of hydrocarbon chloride than said mixture or said firstoverhead, and withdrawing from the bottom of said second fractionator abutadiene product being substantially lower in hydrocarbon chloride thansaid mixture. lt was found in the distillation of the methyl and vinylchloride in the second fractionator that there is a beneficial andunexpected enhancement of the volatility of vinyl chloride relative tothe C hydrocarbons in the fractionator. The present invention isparticularly useful in an integrated process for the preparation ofbutadiene comprising the steps of oxidatively dehydrogenating a feedcomprising n-butenes at an elevated temperature in the presence ofchlorine, oxygen and an oxidative dehydrogenation catalyst, producing aproduct gas comprising butadiene, butenes, condensable gases andhydrocarbon chlorides, condensing the condensable gases, separating saidcondensable gases from said product gas, contacting the product gas withhydrocarbon absorber, stripping said product gas from said absorber,depropanizing said product gas and separating hydrocarbon chlorides fromsaid product gas wherein the improvement comprises passing said productgas from said depropanizing to a fractionator, withdrawing a bottomfraction comprising n-butenes essentially free of butadiene andhydrocarbon chloride, withdrawing a first overhead comprising butadieneand hydrocarbon chloride, said first overhead having a higherconcentration of hydrocarbon chloride than said product gas, passingsaid first overhead to a second fractionator, withdrawing a secondoverhead, said second overhead having a higher concentration ofhydrocarbon chloride than said product gas or said first overhead andwithdrawing from the bottom of said second fractionator a butadieneproduct being substantially lower in nbutenes and hydrocarbon chlorideconcentration than said product gas.

The use of various absorber oils is shown in U.S. Pats. Nos. 3,402,215,3,412,171 and 3,474,155. To the extent necessary to provide supportiveinformation for carrying out the present invention these patents as wellas those previously cited are hereby incorporated by reference.

Butadiene can be prepared by oxidative dehydrogenation of n-butane orn-butenes. The chlorine present in the dehydrogenation zone may beeither elemental chlorine or any compound of chlorine which wouldliberate chlorine under the conditions of the reaction. Suitable sourcesof halogen are such as hydrogen chloride, aliphatic halides, such asmethyl chloride, 1,2-dichloroethane, cycloaliphatic chlorides, ammoniumchloride, sulfuryl chloride, metal chlorides including molten chlorides,and the like. The chlorine may be liberated partially or entirely by asolid source as disclosed in the process of U.S. Pat. No. 3,130,241issued Apr. 21, 1964. Mixtures of various sources of chlorine may beused. The amount of chlorine, calculated as elemental chlorine, may beas little as about 0.0001 or less mole of chlorine per mole of thehydrocarbon to be dehydrogenated to as high as 0.2 or 0.5.

The hydrocarbon to be dehydrogenated is contacted with oxygen in orderfor the oxygen to oxidatively dehydrogenate the compound. Oxygen may befed to the reactor as pure oxygen, as air, as oxygen-enriched air,oxygen mixed with diluents, solid oxidants, and so forth. Oxygen mayalso be added in increments to the dehydrogenation zone. The amount ofoxygen employed may vary depending upon the desired result, such asconversion, selectivity and the number of hydrogen atoms being removed.Thus, to dehydrogenate butane to butene requires less oxygen than if thereaction proceeds to produce butadiene. Normally oxygen will be supplied(including all sources, e.g., air to the reactor) in the dehydrogenationzone in the range of from 0.2 to 2.0 moles per mole of hydrocarbon to bedehydrogenated and for most dehydrogenations this will be within therange of 0.25 to 1.5 moles of oxygen per mole of hydrocarbon.

Preferably, the reaction mixture contains a quantity of steam or diluentsuch as nitrogen with the range generally being between about 2 and 40moles of steam per mole of hydrocarbons to be dehydrogenated andexcellent results have been obtained within the range of about 5 toabout 30 moles of steam per mole of hydrocarbon to be dehydrogenated.Diluents generally may be used in the same quantities as specified forthe steam. These gases serve also to reduce the partial pressure of thehydrocarbon.

The dehydrogenation reaction may be conducted in the absence of contactcatalysts, but better results are obtained if the reaction is conductedin the presence of metal or metal compound catalysts. The previously-cited U.S. patents disclose a number off suitable catalysts.Particularly useful oxidative dehydrogenation catalysts comprise metalferrites wherein the metal is Mg, Zn, Ni, Co, Mn, Cu, Cd, Ca, Ba, Sr,Cr, Ti, V, Mo, W, Na, Li, K, Sn, Pb, Sb, Bi, Ga, Ce, La, Th and mixturesthereof. The dehydrogenation reactor may be a fixed or fluid bedreactor. Reactors such as those conventionally used for thedehydrogenation of hydrocarbons to butadiene may be employed. The totalpressure in the dehydrogenation zone may suitably be about atmosphericpressure. However, higher pressures or vacuum may be used. Pressuressuch as from about atmospheric (or below) up to about to 200 p.s.i.g.may be employed. The dehydrogenation reaction will normally be conductedat a temperature of reaction between about 600F. to about 1,500F. orhigher although generally the maximum temperature in the reactor will bewithin the range of about 700F. and 1,300F. This temperature of thereaction is measured at the maximum temperature in the reactor. The flowrates of the reactants may be varied quite widely and will be dependentsomewhat on whether fixed or fluid bed reactor is employed. Good resultshave been obtained with flow rates of the hydrocarbon to bedehydrogenated ranging from about 54th to 25 liquid volumes ofhydrocarbon to be dehydrogenated per volume of reactor zone per hour,with the volumes of hydrocarbon being calculated as the equivalentamount of liquid hydrocarbons at standard conditions of 15.6C. and 760millimeters of mercury absolute. For the purpose of calculating flows,the reaction zone is defined as the portion of the reactor whichcontains catalyst and which is at a temperature of at least 600F. Inother words, the volume of reaction r-action zone is equivalent to thevolume of the catalyst zone if it were empty. The residence or contacttime of the reactants in the dehydrogenation zone depends on severalfactors involved in the reaction. Contact time such as about 0.001 toabout 5, 10 or 25 seconds have been found to give excellent results.Under certain conditions higher contact times may be utilized. Contacttime is the calculated dwell time of the reaction mixture in thereaction zone assuming the moles of product mixture are equivalent tothe moles of feed.

The effluent from the dehydrogenation zone will contain the impureunsaturated hydrocarbon products, oxygen, various impurities includingoxygenated hydrocarbons, non-condensable inert gases and depending uponthe particular process, some unconverted feed and halogenated compounds.If air was used as the source of oxygen, nitrogen will be present inrelatively large quantities as a non-condensable gas. Steam may bepresent in an amount up to 96 mole percent of the total effluent, suchas from about 5 to 96 mole percent. The organic phase includingdehydrogenated product, any unreacted feed, oxygenated hydrocarbons, anyhalogenated compounds, polymer and tar and precursors thereof. Anyorganic decomposition products usually range from about 3 to 50 molepercent of the effluent and generally will be within the range of about3 to 30 or 35 mole percent of the effluent. The gaseous product willusually contain less than or no greater than 1.0 mole percent oxygenbased on the unsaturated. hydrocarbon such as butadiene. Thenon-condensable gases (under the conditions encountered) such asnitrogen, will be present in an amount of from about 20 to 93 molepercent of the total effluent.

The effluent gases leaving the dehydrogenation zone will generally be attemperature of about or greater than 600F. or 700F. to 1,600F dependingupon the particular dehydrogenation precess. The effluent gases arethencooled prior to further treatment according to this invention. Thereactor effluent may be cooled by any means or combination of means asby quenching followed by employing waste heat boilers, condensers, vaporseparators and the like. Ordinarily, water will be removed as condensedsteam from the gaseous effluent during this cooling operation.

The gaseous effluent is then contacted with an absorber oil such astoluene, benzene, vinyl cyclohexene and the like at a temperature of 60to 150F. at 100 too 200 psig. The fat absorbed oil is heated in aseparator at 55 to 190F. at to 50 psig to remove inert noncondensablegases including oxygen, nitrogen, C0,, the various C s and C andacetylene compounds which are taken off as overheads and disposed of.The fat absorber oil is then sent to a hydrocarbon stripper where theunsaturated hydrocarbon is stripped off and taken overhead. Thisoverhead may then be treated according to the present invention orconventionally depropanized to remove the remaining C s.

The initial feed stream containing butadiene is usually a very complexmixture including, for example, isobutylene, butene-l, butene-2 (bothhigh and low boiling) n-butane, butadiene-1,2, butadiene-1,3, a heavyfraction of C and more carbon atoms and small quantities of both methylchloride and vinyl chloride. The feed stream may have been partiallypurified prior to the present process. For example, the butadiene streamcan be freed of steam and oxygen and depropanized to remove the C andlighter hydrocarbon including a large quantity of methyl chlorideoriginally present. It is to be understood that other materials can bepresent in the butadiene stream to be fractionated and the stream may beless pure or more pure than illustrated. Impurities more volatile thanbutadiene will be carried off in the overhead with the hydrocarbonchlorides whereas impurities less volatile than butadiene will for themost part continue in the butadiene stream or pass of in the bottomspurge. Even with considerable variation in the butadiene feed to thefractionator, the benefits of hydrocarbon chloride removal andconcentration thereof can be achieved according to this invention. Aparticularly useful stream would be one containing at least 35 molepercent butadiene and preferably 45 to 65 mole percent butadiene and 40to 25 mole percent butene-2. After the removal of hydrocarbon chloridethe butadiene stream according to the invention can be further treated,for example, a fractionation to separate butene-2.

The drawing is a schematic representation of one embodiment of thepresent process. The feed stream is predominantly butadiene-1,3 withsmall amounts of butane, butane-2, butene-l and trace amounts of methylchloride and vinyl chloride. The feed stream 1 is fed to fractionator A.Fractionator A is a conventional distillation tower or column and canhave from about 100 to 200 trays. The temperature gradient throughfractionator A will be about 35 to 50F, the bottom temperature being inthe range of 155 to 170F., with lower temperatures in the upper portionof tower. The feed to fractionator A may be preheated by heat exchange(not shown) and will generally be at a temperature in the range of 85 to110F. upon entry into fractionator A. An overhead fraction 2 containingthe butadiene, methyl chloride and vinyl chloride is withdrawn andcondensed in overhead condenser 3 hence to overhead accumulator 4 wherea portion of the butadiene is recovered and returned as reflux 5 tofractionator A. The remainder of the material in overhead accumulator 4is passed as feed 6 to fractionator B. The bottoms 7 from fractionator Aare substantially free of butadiene and chlorides, are high inbutene-2(also contain any C s in the stream) and can be recycled as feedfor the oxidative dehydrogenation. Feed 6 from fractionator A entersfractionator B at a temperature in the range of 40 to 125F. which can beadjusted, if necessary, by heat exchange (not shown). Fractionator B isa conventional distillation tower or column with from 55 to 120 trays.The temperature differential in the tower is about 5 to 15F. preferablyless than 10. The overhead from fractionator B passes to overheadcondenser 9 and then to overhead accumulator 10. A major portion of thematerial in overhead accumulator 10 is returned to fractionator B asreflux. A chloride concentrate 12 is taken off of overhead accumulatorl0 and consists of butadiene and butene-l with a high concentration ofmethyl and vinyl chloride.

The bottoms of fractionator B are the butadiene product which is nowsubstantially free of organic chlorides yet with very little loss inbutadiene product. Normally this butadiene stream is subjected to one ormore extractive distillations to remove less saturated compounds andfurther fractional distillations to separate methyl and vinyl acetylenesand butene-2 to give a final product of 98-99.8 mole percent butadiene.

In addition to the equipment schematically shown in the drawing otherpieces of equipment conventionally employed in fractionations,particularly in regard to the convention aspects of the presentinvention as described may be used, such as reboilers, pumps, coolers,heat exchangers, traps, and the like.

The following example is only illustrative of the invention and is notintended to limit the invention. All percentages are weight percentunless specified otherwise.

EXAMPLE All flow rates were calculated at F. regardless of thetemperature of the stream. The feed is the product of an oxidativedehydrogenation of n-butenes(77.8 liq. vol. percent butene-2, 14.2 liq.vol. percent butene-l, the remainder being principally butane andbutadiene.) Halogen is supplied to the oxidative dehydrogenation as HClin the presence of a metal ferrite catalyst. The condensable gases arefirst removed from product gases from the dehydrogenation zone. Theproduct gases are then contacted with a lean absorber oil such astoluene, stripped from the absorber oil, and C and lighter gasesremoved. Following this preparative procedure a feed stream having thecomposition and flow rates as shown in Table l is fed at F. to adistillation tower having trays. The bottoms temperature is F. and atemperature at the top of the tower of 126F.

TABLE I B/H" Liquid Vol.

Methyl Chloride 5 ppm by mole Vinyl Chloride 10 ppm by mole Butene-l27.54 5.13 Butadiene 328.30 61.13 Butane 31.82 5.93 Butene-2 (lowboiling) 91.48 17.03 Butene2 (high boiling) 57.93 10.78 includes C 's.

barrels per hour The reflux is at 1,300 barrels per hour and corresponds to the composition shown in Table 111.

The bottoms from the fractionator A are free of butadiene and halides asshown in Table 11.

TABLE I1 B/H Liquid Vol.

Methyl Chloride Vinyl Chloride Butene-l tr Butadiene .30 .41 Butane 6.598.91 Butene-2 (low) 33.47 45.27 Butene-Z (high) 33.57 45.41

The overhead fraction not recycled to the fractionator A passes on tofractionator B which is a distillation column with 86 trays. The feed tosecond fractionator has the composition and rates shown in Table III andis The bottoms temperature in fractionator B is about 130F while thetemperature overhead is at 120F. The overhead is condensed and a refluxof 1,250 barrels per hour and corresponding to that shown in Table IV isreturned to fractionator B while a vaporous portion of the overhead fromfractionator B is drawn off containing substantially all of thechlorides as seen in Table IV.

TABLE IV B/l-i Liquid Vol. Methyl Chloride 2.36 mole Vinyl Chloride 1.24mole Butene-l .08 38.10 Butadiene .13 61.90 Butane tr Butene-2 (low)Butene-2 (low) The bottoms from fractionator B have .5 ppm vinylchloride as shown in Table V which is for all practical purposes 0. andwill not interfere with the butadiene subsequent purification or use.

TABLE V B/H Liquid Vol.

Methyl Chloride Vinyl Chloride 0.5 ppm mole Butene-l 27.46 5.93Butadiene 327.87 70.83 Butane 25.23 5.45 Butene-Z (low) 28.01 12.53Butene-2 (high) 24.36 5.26

The loss of butadiene necessitated by the purification to remove theorganic chlorides was only about 0.04 percent based on barrels per hourbutadiene entering fractionator A and barrels per hour butadiene leavingas product from fractionator B. This loss in butadiene product isinsignificant in regard to the substantial increases in yields ofbutadiene in the oxidative dehydrogenation attributable to the presenceof the chloride, e.g., 5 to 15 mole percent absolute or more dependingon CH- concentration, temperature,

catalyst, etc.

The invention claimed is:

1. A process for separating hydrocarbon chlorides from a mixturecontaining butadiene, butenes, and hydrocarbon chlorides comprising (1)passing said mixture to a first fractionator, (2) withdrawing a bottomfraction from said first fractionator, said bottom fraction comprisingn-butenes being essentially free of butadiene and hydrocarbon chloride,(3) withdrawing a first overhead from said first fractionator, saidfirst overhead comprising essentially all the butadiene and hydrocarbonchloride from said mixture, (4) passing said first overhead to a secondfractionator, (5) withdrawing a second overhead from said secondfractionator, said second overhead containing substantially all thehydrocarbon chloride from said first overhead and up to 0.15 percent byvolume of the butadiene in said first overhead and (6) withdrawing fromthe bottom of said second fractionator a butadiene product beingsubstantially free of hydrocarbon chloride.

2. The process according to claim 1 wherein the temperature in thebottoms of the first fractionator is in the range of to F. and thetemperature gradient in said first fractionator is 35 to 50F. and thetemperature in the bottoms of said second fractionator is in the rangeof 45 to 135F. and the temperature gradient in said second fractionatoris 5 to 15F.

3. The process according to claim 2 wherein the said mixture containingbutadiene and hydrocarbon chloride has at least 35 mole percentbutadiene.

' 4. The process according to claim 3 wherein said mixture contains 45to 65 mole percent butadiene and 40 to 25 mole percent butene-2.

5. in an integrated process for the preparation of butadiene comprisingthe steps of oxidatively dehydrogenating a feed comprising n-butenes atan elevated temperature in the presence of chlorine, oxygen and anoxidative dehydrogenation catalyst, producing a product gas comprisingbutadiene, butenes, condensible gases and hydrocarbon chlorides,condensing the condensible gases, separating said condensible gases fromsaid product gas, contacting the product gas with a hydrocarbonabsorber, stripping said product gas from said absorber, depropanizingsaid product gas and separating hydrocarbon chlorides from said productgas wherein the improvement comprises (1) passing said product gas fromsaid depropanizing step to a fractionator, (2) withdrawing a bottomsfraction comprising n-butenes being essentially free of butadiene andhydrocarbon chloride, (3) withdrawing a first overhead from said firstfractionator, said fractionator, said first overhead comprisingessentially all the butadiene and hydrocarbon chloride from said productgas, (4) passing said first overhead to a second fractionator, (5)withdrawing a second overhead from said second fractionator, said secondoverhead comprising essentially all the hydrocarbon chloride from saidfirst overhead and from 0.04 percent to 0.15 percent by volume ofbutadiene in said first overhead and (6) withdrawing from the bottom ofsaid second fractionator a butadiene product being substantially free ofhydrocarbon chloride and substantially lower in n-bu tenes than saidproduct gas.

$27333? TED STATES PATENT oTTTcr fiEll'lllFlCA'iE 'i EQ'llUN' Patent No.3,719, 722 Dated March 6, 1.973

Inventofls) Lloyd D. Tsch opp It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

C01. 3, line 59 reads "off" but should read of Col. 4, line 26 reads"reaction r-action zone" but should read reaction zone Col. 4, line 65reads "precess" but should read process Col. 5, line 38 reads "pass of"but should read pass off Col, 6, line 67 reads under column headed B/ H,last item "57. 93" but should read 57. 93

Col. 7, Table IV, last item in first column reads "Butene2(low)" butshould read Butene-Z (high) Signed and sealed this 15th day of January197M.

(SEAL) Attest:

EDWARD M0T*LE']?CI-IER,JRo I RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents

1. A process for separating hydrocarbon chlorides from a mixturecontaining butadiene, butenes, and hydrocarbon chlorides comprising (1)passing said mixture to a first fractionator, (2) withdrawing a bottomfraction from said first fractionator, said bottom fraction comprisingn-butenes being essentially free of butadiene and hydrocarbon chloride,(3) withdrawing a first overhead from said first fractionator, saidfirst overhead comprising essentially all the butadiene and hydrocarbonchloride from said mixture, (4) passing said first overhead to a secondfractionator, (5) withdrawing a second overhead from said secondfractionator, said second overhead containing substantially all thehydrocarbon chloride from said first overhead and up to 0.15 percent byvolume of the butadiene in said first overhead and (6) withdrawing fromthe bottom of said second fractionator a butadiene product beingsubstantially free of hydrocarbon chloride.
 2. The process according toclaim 1 wherein the temperature in the bottoms of the first fractionatoris in the range of 155* to 170*F. and the temperature gradient in saidfirst fractionator is 35* to 50*F. and the temperature in the bottoms ofsaid second fractionator is in the range of 45 to 135*F. and thetemperature gradient in said second fractionator is 5* to 15*F.
 3. Theprocess according to claim 2 wherein the said mixture containingbutadiene and hydrocarbon chloride has at least 35 mole percentbutadiene.
 4. The process according to claim 3 wherein said mixturecontains 45 to 65 mole percent butadiene and 40 to 25 mole percentbutene-2.